Devices which perform hardening using a laser beam generally comprise an optical mechanism consisting of a laser oscillator, reflecting mirror and condensing lens, and shield gas supply system for excluding oxygen from the irradiated points.
Before an object to be hardened is irradiated by a laser beam, the surface of the object is coated with a laser absorbing agent to increase the laser absorption of the object and form a uniform hardened layer on the object. After irradiation, the coating is removed.
The laser is generally a CO.sub.2 laser, and the laser absorption agent may or example comprise the components in the Table 4.1 "CO.sub.2 absorption coating material table" on page 77 of "Laser Kakou Gijutsu" written by Hiromichi Kawasumi (Nikkan Kogyo Shinbunsha, 28 Jan., 1985).
If parts subject to hydraulic pressure, e.g. parts for automatic transmissions in automobiles, are to be hardened, it is desirable that oxide films or other impurities are not present on the surface so as to prevent deterioration of oil performance. Laser absorption agents prevent such oxidation of the object surface which would otherwise be caused by the laser irradiation.
Laser absorption agents therefore have a desirable effect on the formation of a hardened layer, but their application and removal require much effort which increases the cost of hardening.